Testing of these values is typically carried out at a relatively warm temperature (70°F [21°C]) and in static conditions, with no air movement. It is important to remember that the performance of many insulation types degrades as the temperature drops and air moves around or through the material.
Thickness — The effectiveness of insulation is proportional to the thickness of the material. R-value is commonly expressed as a value per inch of material thickness (R/inch).
Thermal bridges occur anywhere a high conductivity material connects the inside and outside. Minimizing thermal bridges is critical to thermal performance. Continuity and thermal bridging — The overall insulation value of a building enclosure is determined by the thermal conductivity of the entire surface area of the building. If there are thermal bridges in the building enclosure, the increased heat movement in those areas reduces the overall thermal performance proportional to the surface area of the thermal bridges. Windows and doors — Openings (fenestra-tions) in buildings are an integral part of the thermal control layer, representing large areas of reduced insulation value. There are significant differences in thermal performance between competing windows and doors, and the greater the AT in the installation environment and the larger the overall area of fenestration, the greater the effect on the overall performance of the building’s thermal control layer.
Effects of convection — The transfer of heat by convection can have a significant impact on the performance of insulation if air is able to travel freely inside or around the insulation. To eliminate poor performance due to convection:
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Ensure a continuous air barrier (see Air Control Layer, page 72) to prevent heat loss due to air movement through the thermal control layer.
Install insulation materials so there are no gaps or voids between the insulation and other elements of the enclosure. This can be particularly challenging when installing batt or rigid insulation around interruptions like framing, wiring, blocking, and other obstructions.
Floor framing systems touching the outside of the enclosure can be a point of major thermal losses and air movement.
Effects of radiation — Radiation, especially from direct exposure to sunlight, can have a significant impact on thermal performance. In areas where radiant gains from the sun need to be controlled, the thermal control layer may include: